NADH cytochrome b5 reductase 3 (Cyb5R3: NADH:ferricytochrome b5 oxidoreductase, EC1.6.2.2) or methemoglobin reductase is a flavoprotein known for its ability to transfer electrons from the NADH domain of Cyb5R3 to cytochrome b5. Membrane restricted Cyb5R3 in somatic cells regulates several biological reduction reactions including elongation and unsaturation of fatty acids1, cholesterol biosynthesis and drug metabolism, while the soluble form Cyb5R3 resides in erythrocytes to reduce methemoglobin. In the human population, deficient Cyb5R3 activity leads to recessive hereditary methemoglobinanemia (RHM). Type I RHM displays mildly elevated methemoglobin levels in erythrocytes whereas Type II RHM, which affects all somatic cells, causes severe developmental neurological disorders. Recent evidence suggests that membrane-bound Cyb5R3 expression and activity also contribute to metabolic homeostasis, stress protection and nitric oxide (NO) bioavailability.
Within the vascular wall, the importance of Cyb5R3 in the endothelium has gained appreciation for its role in NO signaling. NO, a naturally produced biogas, contributes to diverse biological processes and is well-known known for its role as a potent vasodilator. Recent evidence revealed α globin expression in small artery and arteriolar endothelial cells where it regulates NO signaling. Enriched in the myoendothelial junctions—the anatomical location where endothelium and vascular smooth muscle make contact—α globin controls NO diffusion to vascular smooth muscle. This process is carried out via biochemical reactions of NO with α globin, whereby synthesized NO from endothelial oxide synthase can react with oxygen bound ferrous heme iron (Fe2+) α globin resulting in NO scavenging. However, ferric heme iron (Fe3+) α globin permits NO diffusion through a slow and weak reaction. Serving as a switch to control the heme iron redox state of α globin, Cyb5R3 modulates NO bioavailability and thus arterial vascular tone. Therefore, Cyb5R3 serves as an attractive biological target to increase NO bioavailability in order to augment microcirculatory blood flow and decrease blood pressure in the setting of cardiovascular disease.
Currently, there are no potent small molecule inhibitors that block Cyb5R3 activity. Previous studies demonstrated that propylthiouracil (PTU), a drug designed to treat hyperthyroidism, showed inhibition of Cyb5R3 at a concentration of approximately 275 μM. Nonetheless, it is still unclear how PTU exerts its inhibitory effects at a molecular level. By understanding the mechanistic action by which PTU inhibits Cyb5R3, a new class of inhibitors could emerge.